Media Forward Link Only (also referred to herein as MediaFLO or FLO) and DVB-H systems provide for hierarchical data channels created by operating different data streams on respectively differing portions of the modulation signal constellation. As shown in FIG. 1, the base channel (or base modulation layer) is represented by the upper two bits of a 16 QAM constellation, while the enhancement channel (or enhancement modulation layer) is represented by the lower two (least significant) bits. This exemplary scheme results in 1 bit in the enhancement layer for each bit in the base layer, a 1:1 ratio. Other constellations maybe divided into other sets of layers, but all hierarchical systems based on fixed allocation of modulation states to a layer will result in a fixed data rate ratio among the various layers. The above-described use of hierarchical data channels is also referred to herein as layered modulation. For a FLO system, a Multicast Logical Channel (MLC) that utilizes the hierarchical data channels is referred to herein as a layered MLC. An MLC that does not employ hierarchical data channels is referred to herein as a non-layered MLC.
The aforementioned fixed data rate ratio between the N available channels can cause inefficiencies when used with codecs that produce layered source coding. For example, temporal based layering for video codecs such as H.264 can result in streams with close to 1:1 bit rate proportions for high motion content; however the ratio of bit rates for low motion content can be highly imbalanced. Similarly, stereo audio may be coded as L+R and L−R, however the L+R signal on average may contain twice as much data as the L−R signal. There are conditions under which the audio data rates can be balanced, but it is not the nominal condition. There is accordingly a need for variable bit rate ratio among the respective modulation layers, and variable bit rate overall. Although existing systems such as MediaFLO and DVB-H allow for variable bit rates, their hierarchical layered modulation schemes structurally result in the ratio of the bit rates being integer or rational fractional values.